In the intense search for high capacity anode materials for use in lithium-ion rechargeable battery technology, researchers have focused their efforts on the study of various carbonaceous materials. Graphite, because of its ability to rapidly and reversibly intercalate one lithium per six carbons (yielding a charge capacity of 372 mAh/g) at a voltage &lt;0.5V vs. Li, is presently used as the anode material in Li-ion batteries. Nevertheless, an irreversible discharge plateau near 0.8 V, ascribed to electrolyte decomposition, appears during the first discharge of C/Li cells and its magnitude, i.e., irreversible capacity, is a function of both the nature of the graphitic carbon as well as of the electrolyte used.
Recently, there have been numerous reports on various types of carbonaceous battery electrode materials, including the so-called disordered hard carbons which display capacities up to twice greater than that of graphite, with however a large irreversible loss (&gt;20%) between the first discharge and the first charge. Despite such a large loss, for several reasons (better cycle life, less bulging, less risk of Li plating and easier cell balancing), this type of carbon is useful, as well as graphite, for use in commercial Li-ion cells. A comprehensive review of the structure and utility in electrolytic cells of a broad range of investigated carbon materials, from natural "perfect" graphite to coke and soft and hard carbons specifically synthesized for the purpose, as by pyrolysis of selected organic precursor compounds, has been published by J. R. Dahn et al. in Lithium Batteries--New Materials, Developments and Perspective, edited by G. Pistoia (Elsevier North Holland) Chapter 1, pp. 1-47 (1993).
The irreversible cell capacity loss is believed to be mainly due to electrolyte decomposition that results in the formation of a passivating film or Solid Electrolyte Interphase (SEI) on the carbon surface. Numerous studies have examined the chemical composition and formation of SEI, and new electrolyte compositions leading to only about 7% irreversible losses in some types of graphite have been found. However, with the same electrolytes other graphitic carbons have shown losses as high as 25%. The fact that these various carbon materials differ in such properties as morphology, specific surface area, and defects density makes any systematic study of SEI formation difficult. Due to such limitations, the role played by carbon material surface states in SEI formation and resulting irreversible capacity loss has been unclear.
The discoveries underlying the present invention have arisen from extensive investigations by the inventors into the comminuting, or pulverizing, of carbon materials by mechanical grinding and the effect of such size-reduction operations on crystallite size, morphology, specific surface area, and electrochemical performance of the resulting carbon materials when used in electrodes of Li-ion/carbon electrolytic battery cells. As a result of the comminuting treatment of the present invention, a wide range of carbonaceous precursors of differing properties and morphology may be employed to consistently obtain carbon electrode materials which are capable of reversibly intercalating about two lithiums per six carbons, i.e., Li.sub.2 C.sub.6, to yield reversible capacity of 708 mAh/g while having irreversible capacity of only about 328 mAh/g.